The present invention relates to display, in particular to a liquid crystal display illuminated by ambient light.
Some displays, such as liquid crystal displays, can be lit by a back light. The back light may need to be powerful, particularly if the optical efficiency of the display is low. Therefore, if the back light is part of a lap top computer or other portable device, a heavy battery may be required to power the back light, which may be inconvenient.
Displays are known that operate without a back light by reflecting ambient light, the reflectivity of the display being different at different points on the screen, so that reflected light forms an image. However, a substantial amount of ambient light incident on the display may be lost due to absorption within the display. Furthermore, the user of the display may be tempted to point the display in the direction where the ambient light is strongest, which will lead to specular reflection of the ambient light off the display, thereby making the display less effective.
Another type of display which uses a planar waveguide to provide light through a liquid crystal layer is disclosed in a paper titled xe2x80x9cWaveguide Based Liquid Crystal Displayxe2x80x9d, by H. Yuan and P. Palffy-Muhoray, published in Mol. Cryst. Liq. Cryst., 1999, Vol. 331, pp.281-288. This uses total internal reflection between the liquid crystal and one planar substrate that bounds the liquid crystal, instead of birefringence between crossed polarizers. In this planar waveguide lit display, one or more light sources must then be provided to couple light into the waveguide, for example at the periphery of a display. Light is then guided by the substrate and then deflected out of activated areas of the liquid crystal. This provides an efficient optical structure with a high viewing angle. It may, however, be difficult to provide one or more light sources around the periphery of the display to illuminate the waveguide.
According to one aspect of the present invention, there is provided a display having a display area, and a light guide for providing light to the display area, the light guide comprising:
a planar light guiding medium having a light emitting surface and one or more side faces disposed around the light emitting surface;
a plurality of light pipes, each light pipe having a collector end for collecting light and an output end, the output ends being arranged along the side faces so as to introduce the collected light into the guiding medium;
wherein the output ends of the light pipes are distributed along the or each side face.
The light pipes may then be distributed according to the amount of light needed across the display, or in a particular area of the display. In a preferred embodiment of the invention, the light pipes are distributed evenly along the or each side face.
The light guiding medium will channel light therealong but will allow some light to escape through the light emitting surface to illuminate the display area. It will be understood that the term planar light guiding medium will include a medium with some curvature, provided that the curvature is sufficiently small that to allow the guiding medium to channel light therein.
Because the output ends of the light pipes are distributed evenly along the side faces of the light guiding medium, light will be emitted from the light emitting surface more evenly. It will be appreciated that the light pipes outputs need not be distributed at the same intervals around all the side faces of the light guiding medium, and that if the light guiding medium is rectangular, the intervals between light pipe outputs along the long sides may be different from the intervals along the short sides of the light guiding medium.
Preferably, the collector ends of the light pipes will be distributed in a spaced apart fashion over a light collecting area, the position of the collector ends on the light collecting area being scrambled relative to the position of the corresponding output ends on the side faces of the light guiding medium. Because of this, if a portion of the light collecting area is obscured, the light emitted through the light emitting area will be diminished in a more uniform manner, and the likelihood of a shadow being formed in the display area will be reduced.
There may be some short range order between the positions of the output ends and the positions corresponding light collector ends. The light pipes may be grouped in pairs, the light output ends of each pair being adjacent to one another on a side face, and the collector ends of each pair being adjacent to one another on the collecting area. However, a group of light pipes with neighbouring or nearby collector ends will preferably have output ends that are located on different portions of a side surface or, on different side surfaces.
The light guiding medium will preferably have a planar back surface, and a reflecting layer may be provided on the back surface of the light guiding medium, so as to increase the amount of light emitted through the light emitting surface.
In a preferred embodiment, means are provided for applying an electrical signal to the guiding medium in one or more localised areas, and the guiding medium is responsive to the electrical signal such that the optical properties of the optical medium are changed in each localised area where the electrical signal is applied, with the result that in the localised areas where the electrical signal is applied, light travelling along the guiding medium exits the guiding medium through the light emitting surface, and where the electrical signal is not applied, light within the light guiding medium is channelled therealong.
An image can be formed by applying the electrical signal at the points where the desired image is light, the remaining area of the display appearing dark.
The electrical signal will preferably be in the form of a voltage applied across the guiding medium, but the electrical signal may superposed on a background voltage level, such that the electrical signal at a point on the light guiding medium results in the absence of a voltage being applied at that point.
The guiding medium may have a refractive index whose value is changeable between a first value and a second value when an electrical signal is applied, the guiding medium having an index matching layer whose refractive index is close to the first value. In one embodiment, the refractive index of the guiding medium will be more closely matched to the refractive index of the index matching layer at a point where an electrical signal is applied, and light will be emitted through the light emitting layer at that point. The light guiding medium will preferably be liquid crystal material, since its refractive index for at least one polarisation of light can easily be changed by applying an electric field.
So that the image produced by the screen can be viewed more easily by a person in front of the screen, a light scattering layer will preferably be provided between the light emitting surface and the light guiding layer.
Since the liquid crystal material will normally be birefringent, and since the refractive index for only one polarisation of light may be changed between a fist value and a second value by the electrical signal, a quarter wave plate or other polarisation rotating element may be provided at each side face of the light guiding medium, in order to increase the amount of light which can be affected by the change in refractive index of the liquid crystal material. A semi-reflecting mirror will preferably be provided on the outer surface of each quarter wave plate, so that light of one polarisation that is internally incident on a side face will pass through a quarter wave plate a first time, will be reflected by the semi-reflecting mirror, and will then pass through the quarter wave plate a second time, emerging from the quarter wave plate with a second polarisation.
The collector ends of at least some of the light pipes may be secured together in a bunch having a collector face, and means may be provided for temporarily securing the bunch in an orientation relative to the display area. If the ambient light from a particular direction is particularly bright, a user will then be able to orient the bunch such that the connector ends face in the direction where the ambient light is bright, thereby increasing the brightness of the screen. Alternatively, the end of the bunch may be optically coupled to an electrical light source.
In a preferred embodiment, the display will be secured in a casing, and the collector ends will be distributed over at least a portion of the external surface of the casing. The display may be secured to the casing of a lap top computer, or other portable electronic device of the type having a display, such as a mobile telephone.
The light pipes will preferably be formed from optic fibres.